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0 1 2 km
Fig. 1. Bathymetry of Lake Rotorua, including elevation of the surrounding landscape.
Note the historical lake bed (shaded mauve), the current lake extent (delineated by the
brown-shaded region of coarse sediments within the historical lake bed) and the transition
to fine sediments of the inner lake basin (blue regions in the interior). Mokoia Island is
readily distinguished near the centre of the present lake basin.
Pock marks and methane eruptions
Early attempts to use seismic techniques to survey the
stratigraphy of the lake sediments (Davey, 1992) were
unsuccessful due to the absorption of acoustic signals by
gas within the sediments. This gas was interpreted to be
of geothermal origin. A recent seismic survey (Pearson,
2007) also showed no structural features of the lake
sediments except where there are small, circular, flatbottomed
depressions, detectable in Fig. 1 as minute
depressions in the diatomaceous ooze of the deeper bed
of the lake. The depressions are typically steep-sided and
up to 5 m deeper than the surrounding lake bed, and
Fig2. Gas erupting on the
around 50 m in diameter. Similar circular depressions surface following sediment
are common in shallow marine and lacustrine sediments
and have been termed pockmarks, with their formation
disturbance by gravity corer.
interpreted as a result of gas discharge (Rodgers et al.,
2006). Evidence of ebullition of gas can commonly be observed at the surface of Lake Rotorua
on calm days (Fig. 2), particularly over pockmarks, and also in sediment cores raised to the
lake surface (Pearson, 2007). Gas captured from beneath the water surface was a mixture of
GSNZ Newsletter 143 (2007) Page 8